Cam Question For GM 847 Users?
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From: Washington, Michigan USA
Cam Question For GM 847 Users?
I'm using an internet "dynamic" compression ratio calculator to model an LT1 engine I'm thinking of building, and need to know the intake valve closing point in "degrees ABDC at .050" plus 15 degrees", as an input for the specific DCR calculator I'm using, with the GM 847 cam I'm modeling in my projected build. I know the GM 847 cam is a pretty popular option for the LT1 set so how about it guys, anyone know the IVC point at .050" for the GM 847 cam? (And yes, I can add the 15 degrees to the value you give me to get the correct input for the calculator 
) TIA for any replies!
) TIA for any replies!
Last edited by 97 6SPEED Z; Apr 21, 2006 at 12:47 PM.
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Posts: 2,182
From: Moore OK
Re: Cam Question For GM 847 Users?
I am no expert, but everything I have read says the +15 is wrong. The calculations on the Keith Black site are correct however.
For the 847, I calculated with another program the ABCD to be..... one sec have to find it
ABCD 76 degrees (use that in your dynamic calculator)
This is assuming
296 intake, 304 exhaust, 112 lsa and 108icl.
This is based on the best data I could find with many hours of searching. Some have said the icl is 107 with +5 making 112 lsa. If so that would only lower the ABcd to 75.
I switched from the cc306 to the gm847 and I think it lowered my dynamic @ .3. I am running 12.0:1 static, so the 847 was about as small as I could go and keep below 9.0 on my dynamic.
Hope this helps.
For the 847, I calculated with another program the ABCD to be..... one sec have to find it
ABCD 76 degrees (use that in your dynamic calculator)
This is assuming
296 intake, 304 exhaust, 112 lsa and 108icl.
This is based on the best data I could find with many hours of searching. Some have said the icl is 107 with +5 making 112 lsa. If so that would only lower the ABcd to 75.
I switched from the cc306 to the gm847 and I think it lowered my dynamic @ .3. I am running 12.0:1 static, so the 847 was about as small as I could go and keep below 9.0 on my dynamic.
Hope this helps.
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From: Cincinnati, Ohio
Re: Cam Question For GM 847 Users?
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Re: Cam Question For GM 847 Users?
First off, jsetzer and atljar thanks indeed for both your replies. 
Using the information supplied by both of you for the GM 847 cam does, of course, yield wildly varying results. Jsetzer you say to use 76 degrees ABDC, (without adding the additional 15 degrees), and atljar the cam card you linked shows 44 degrees ABDC @ .050", which when I add 15 degrees to it, (as requested by my DCR calculator), I get a value of 59 degrees ABDC. Needleess to say when I then plug either 76 or 59 degrees ABDC into the calculator, (without changing any other of my engine specs), I get DCR's of 8.477 and 9.770, respectively.
Since I'm shooting for something in the 8.9 to 9.0 range, one calculation says I'm too low at 8.477 and one says I'm way too high at 9.770. 
Any further "feel" as to which one of these values, (i.e. either 76 or 59 degrees ABDC) may be correct?
P.S. Also, anyone got a link to that Keith Black site mentioned above?
Using the information supplied by both of you for the GM 847 cam does, of course, yield wildly varying results. Jsetzer you say to use 76 degrees ABDC, (without adding the additional 15 degrees), and atljar the cam card you linked shows 44 degrees ABDC @ .050", which when I add 15 degrees to it, (as requested by my DCR calculator), I get a value of 59 degrees ABDC. Needleess to say when I then plug either 76 or 59 degrees ABDC into the calculator, (without changing any other of my engine specs), I get DCR's of 8.477 and 9.770, respectively.
Since I'm shooting for something in the 8.9 to 9.0 range, one calculation says I'm too low at 8.477 and one says I'm way too high at 9.770. Any further "feel" as to which one of these values, (i.e. either 76 or 59 degrees ABDC) may be correct?
P.S. Also, anyone got a link to that Keith Black site mentioned above?
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Joined: Feb 2003
Posts: 2,639
From: Mooresville, NC
Re: Cam Question For GM 847 Users?
You need to use the .004 in the link Atljar gave you.
................Opens.............Closes
Intake........37 BTDC.........79 ABDC
79º is what you need to use!
................Opens.............Closes
Intake........37 BTDC.........79 ABDC
79º is what you need to use!
Last edited by marshall93z; Apr 22, 2006 at 04:32 PM.
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From: Washington, Michigan USA
Re: Cam Question For GM 847 Users?
Marshall93z, okay I'm begining to see a pattern here! What both you and jsetzer are telling me is that despite the fact that the calculator I'm using is specifically asking for the IVC closing point at .050" lift, (to which it asks you to add 15 degrees), that it really doesn't want that input.
Well if I use the 79 degrees ABDC figure you suggest for the 847 cam's IVC point, then I get a DCR of around 8.4 and I definately should be able to run this modeled engine on 93 octane premium gas with that cam. Thanks to all who replied!
What both you and jsetzer are telling me is that despite the fact that the calculator I'm using is specifically asking for the IVC closing point at .050" lift, (to which it asks you to add 15 degrees), that it really doesn't want that input. Well if I use the 79 degrees ABDC figure you suggest for the 847 cam's IVC point, then I get a DCR of around 8.4 and I definately should be able to run this modeled engine on 93 octane premium gas with that cam.
Thanks to all who replied!
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